Coil springs applied to vehicles have been produced with a high stress of about 120 K in a recent vehicle industry. For example, the coil springs with a high stress of about 130 K have been also massively applied to vehicles. In addition, as a material with a high strength of 110 K to 130 K has been generally applied, the thickness of wire/the number of coil turns may be decreased and thus the weight of vehicles may be reduced. However, after chipping/painting exfoliation, sensitivity to corrosion may increase. In addition, design margin may not be secured due to thickness decrease of the wire, whereby there are risks such as strength deficiency and progression speed acceleration until being reached complete breakage during breakage progress.
In the related arts, in order to reduce such risks, dual coating or the like has been applied only to some parts vulnerable to corrosion. However, excessive material (paint) cost may increase and a fundamental solution may not be provided. Accordingly, durability increase through enhancement of such strength/corrosion problems of a material is a problem that the current vehicle industry must solve. Recently, since vehicles have high performance, high output and high efficiency, high strengthening and weight reduction of components are required. In addition, since steel materials for a suspension should be weight-reduced under conventional vehicle load/corrosion conditions, rigidity and durability of a material should be essentially secured.
The above disclosed background art has been provided to aid in understanding of the present invention and should not be interpreted as conventional technology known to a person having ordinary skill in the art